A variety of print methods have been employed for imaging various sheet materials. Commonly employed print methods include gravure, off-set, flexographic, lithographic, electrographic, electrophotographic (including laser printing and xerography), ion deposition (also referred to as electron beam imaging (EBI)), magnetographics, inkjet printing, screen printing, and thermal mass transfer. More detailed information concerning such methods is available in standard printing textbooks.
One of ordinary skill in the art appreciates the differences in these various print methods and recognizes that a combination of ink and receiving substrate that results in high image quality in one printing method often exhibits an entirely different image quality with another print method. For example, in contact printing methods such as screen-printing, a blade forces the ink to advance and wet the receiving substrate. Image defects are typically due to a subsequent recession of the ink contact angle with the substrate. In the case of non-contact printing methods, such as inkjet printing, the individual ink drops are merely deposited on the surface. In order to achieve good image quality, the ink drops need to spread, join together, and form a substantially uniform, leveled film. This process requires a low advancing contact angle between the ink and the substrate. For any given ink/substrate combination, the advancing contact angle is typically significantly greater than the receding contact angle. Accordingly, ink/substrate combinations that result in good image quality when printed with contact methods such as screen printing, often exhibit insufficient wetting when imaged with non-contact printing methods such as inkjet printing. Insufficient wetting results in low radial diffusion of the individual ink drops on the surface of the substrate (also referred to as “dot gain”), low color density, and banding effects (e.g., gaps between rows of drops).
Another important difference between screen-printing and inkjet printing is the physical properties of the ink. Screen printing ink compositions typically contain over 40% solids and have a viscosity of at least two orders of magnitude greater than the viscosity of inkjet printing inks. It is not generally feasible to dilute a screen printing ink to make it suitable for inkjet printing. The addition of large amounts of low viscosity diluents drastically deteriorates the ink performance and properties, particularly the durability. Further, the polymers employed in screen printing inks are typically high in molecular weight and exhibit significant elasticity. In contrast, inkjet ink compositions are typically Newtonian.
Inkjet printing is emerging as the digital printing method of choice due to its good resolution, flexibility, high speed, and affordability. Inkjet printers operate by ejecting, onto a receiving substrate, controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, inkjet printers can produce a wide variety of printed features, including text, graphics, holograms, and the like. The inks most commonly used in inkjet printers are water-based or solvent-based. Water-based inks require porous substrates or substrates with special coatings that absorb water.